1. Field of the Invention
The present invention relates to a color converting method in which a color is arbitrarily converted to another within a real time by inputting color image signals or color picture signals in a color scanner requiring color correction at high speed, a color camera, a color hard copy equipment, a color displaying equipment requiring an accurate color correction, a color corrector for converting colors of a video picture within a real time, a video editing equipment, and a color recognition equipment for recognizing picture images by distinguishing colors of the images.
2. Description of the Related Art
In a conventional monochromatic picture processing, a piece of information of a monochromatic picture displayed in a pixel is a piece of one-dimensional information of lightness, and a lightness conversion is performed as a gamma curve conversion. Therefore, in cases where various non-linear curves are stored in a look-up table to perform the gamma curve conversion, a color conversion in the pixel can be performed within a real time by converting the lightness of the pixel according to the gamma curve conversion. In addition, in cases where a full color image (or a colored picture) is displayed in a pixel, the full color image has been conventionally treated as a combination of three monochromatic pictures such as a red (R) plane, a green (G) plane and a blue (B) plane, and a look-up table is prepared for each of the monochromatic pictures. Therefore, even though a color conversion of the colored picture is required, the color conversion can be performed within a real time by independently referring the three look-up tables. In this case, the above conventional color conversion is originally a one-dimensional processing, and the transformation of EQU R'=f.sub.R (R), G'=f.sub.G (G), and B'=f.sub.B (B)
can be merely performed to convert the full color image.
However, a piece of information of a full color image displayed in the pixel is a three-dimensional information expressed by (R,G,B) in a real picture image. Therefore, the color conversion to be performed in the full color image is formulated according to an equation (1). EQU R'=f.sub.R (R,G,B), G'=f.sub.G (R,G,B), and B'=f.sub.B (R,G,B)(1)
In the equation (1), the symbols R, G and B denote a red, a green and a blue in a color scanner, and the symbols R', G' and B' denote another red, another green and another blue in a printer or a display. Or, the symbols R. G and B denote a red, a green and a blue in a RGB color space, and the symbols R', G' and B' denote a cyan (C), a magenta (M) and a yellow (Y) in a CMY color space, a lightness, a first chromaticity and a second chromaticity in a lightness-chromaticity color space, or the like.
For example, a complicated color conversion, in which chromaticness of a color belonging to a particular hue is heightened, is required in a full color image processing of a hard copy equipment. Also, a request of a complicated color conversion of a particular color, in which a color of a background is selectively converted to an achromatic color, is required in a full color image editing processing such as a video picture. These color conversions belongs to a three-dimensional color conversion, and three input signals are required to obtain an output signal.
In cases where a color correction (or, a color conversion) of a full color image displayed on an image plane or printed on a paper is performed by inputting a combination of the colors R, G and B, the colors R', G' and B' determined by a composition of the colors R, G and B are respectively stored in a look-up table of a table memory in advance over the entire color space. Thereafter, when a combination of input colors R, G and B is input to correct the color picture, correcting colors R', G' and B' determined by a composition of the input colors R, G and B are respectively read out from the look-up tables. For example, in cases where a full color image read by a color scanner is converted and displayed on an image plane, intensities of three primary colors such as red (R), green (G) and blue (B) are detected by the color scanner in a RGB color space to obtain three color signals. Thereafter, a color conversion is performed according to the equation (1) in a conventional color converting apparatus to convert the color signals to desired color signals in the same RGB color space. Thereafter, a color converted picture formed with the desired color signals is displayed on the image plane. Also, in cases where a full color image read by a color scanner is converted and printed by a printer, three color signals are obtained with the color scanner in the RGB color space. Thereafter, the color signals in the RGB color space are transformed to three transformed color signals of cyan (C), magenta (M) and yellow (Y) in a CMY color space. Thereafter, a color conversion is performed according to the equation (1) in a conventional color converting apparatus to convert the transformed color signals to desired color signals in the CMY color space. Thereafter, a color converted picture formed with the desired color signals is printed on a paper by the printer. The reason that the RGB color space is converted to the CMY color space is because three primary colors of toner are cyan (C), magenta (M) and yellow (Y).
In addition, a standard color space such as a CIE-LAB color space has been recently proposed to standardize various color spaces for convenience. The CIE-LAB color space is formed with an L-axis indicating a lightness of a color, an A-axis indicating a first chromaticity of the color and a B-axis indicating a second chromaticity of the color. Therefore, in cases where a full color image read by a color scanner is converted and printed by a printer, three color signals are obtained with the color scanner in the RGB color space. Thereafter, the color signals in the RGB color space are transformed to three transformed color signals of the lightness, the first chromaticity and the second chromaticity. Thereafter, a color conversion is performed in a conventional color converting apparatus to convert the transformed color signals to desired color signals in the CIE-LAB color space. Thereafter, the desired color signals in the CIE-LAB color space is transformed to desired transformed color signals in the CMY color space, and a color converted picture formed with the desired transformed color signals is printed on a paper by the printer.
However, in cases where the color conversion of the colored picture is performed with general-purpose look-up tables, a memory capacity of 16 Mbytes (2.sup.8 .times.2.sup.8 .times.2.sup.8) is required to convertone of the three types of colors on condition that the input signals indicating the input colors R, G and B respectively have an 8-bit length to store each of the correcting colors R', G' and B' in 2.sup.8 types of densities in the look-up tables. Therefore, a color converting method is required to determine most of the correcting colors R', G' and B' with a small number of referential correcting colors R', G' and B' stored in the look-up tables in advance according to an interpolation process. That is, a three-dimensional color conversion of a full color image can be arbitrarily performed in general-purpose and within a real time according to the color converting method. Also, a small-sized color converting apparatus in which a hardware having a small capacity of memory is provided to perform the color converting method is required.
As an example of a conventional color converting method, a table look-up method utilizing a three-dimensional interpolation has been conventionally proposed to simply convert various complicated color signals at a high speed in a color printing field and a color hard copy field. In the three-dimensional interpolation of the table look-up method, the RGB color space is partitioned into a plurality of partitioned solid units, an input color composed of RGB components is input, a partitioned solid unit including the input color is selected from among the units in the RGB color space, an output color of cyan, magenta or yellow is determined in the CMY color space by interpolating the output color with a plurality of output values of cyan, magenta or yellow in a plurality of vertexes of the selected unit to arbitrarily perform a color conversion with the input color over the entire color space while keeping a continuous color conversion. For example, various types of interpolations such as an 8-point interpolation in which the color space is partitioned into a plurality of cubes, 6-point interpolation in which each of the cubes is moreover partitioned into two triangular prisms, a 5-point interpolation in which each of the cubes is moreover partitioned into three pyramids, and a 4-point interpolation in which each of the cubes is moreover partitioned into five or six tetrahedrons is well-known.
The reason that the various interpolations have been proposed one after another is that a plurality of drawbacks exist in the 8-point interpolation which is the most general three-dimensional interpolation. For example, it is apparent that a large amount of calculating time and a large size of hardware are required in the 8-point interpolation. Moreover, a drawback in another important performance exists in the 8-point interpolation as follows. In cases where primary colors such as a cyan (C), a magenta (M), a yellow (Y) and a black (BK) are produced by utilizing a red (R), a green (G) and a blue (B) according to the 8-point interpolation in a color hard copy field, a look-up table for the black BK is, for example, prepared according to a minimum calculation method in which the red, the green and the blue respectively having a density MIN(Dr, DK, Db) equal to a minimum value of three input color density (or lightness) values (Dr, Dg, Db) of a red input signal, a green input signal and a blue input signal are mixed to produce the black BK. However, because the interpolation in the minimum calculation method is most difficult among those in various non-linear translations in which the translations are generally difficult. Therefore, in cases where the 8-point interpolation is performed to prepare the look-up table for the black BK, the black is interpolated in a wave form having concave slack portions while keeping the continuity of the interpolation, and ripples are generated. As a result, false outlines are formed in a full color image which is colored by mixing the cyan (C), the magenta (M), the yellow (Y) and the black (BK). The false outlines are visually unbearable for a viewer.
In contrast, the minimum calculation method can be performed without any ripple according to the 4-point interpolation utilizing the tetrahedrons or the 5-point interpolation utilizing the pyramids. For example, the minimum calculation method according to the 4-point interpolation is described in Published Unexamined Patent Application No. 226867 of 1990 (H2-226867), and the minimum calculation method according to the 5-point interpolation is described in detail in Published Unexamined Patent Application No. 14237 of 1981 (S56-14237). In the above applications, it is insisted that the interpolation of the black BK directed in an achromatic color (or gray) direction in a color space can be preferably performed because boundary lines partitioning each of the cubes into the tetrahedrons or the pyramids are directed in diagonal axis directions of each of the cubes. This insistence is not sufficient because an interpolation error in the achromatic color direction is merely described. To be accurately described, the color conversion according to the minimum calculation method can be linearly performed without any ripple by performing the interpolation of the black BK directed not only in the achromatic color direction but also other all directions because the boundary surfaces partitioning each of the cubes into the tetrahedrons or the pyramids include all of differential discontinuous surfaces in which results of the minimum calculation in the method become constant in the color space. Therefore, the generation of the ripple in the 4-point interpolation and the 5-point interpolation is technically solved in the interpolation directed in the all directions in the color space according to the minimum calculation method.